When we see records being broken and unprecedented events such as this, the onus is on those who deny any connection to climate change to prove their case. Global warming has fundamentally altered the background conditions that give rise to all weather. In the strictest sense, all weather is now connected to climate change. Kevin Trenberth

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Sunday, November 30, 2008

Earth's climate appears to be changing more quickly and deeply than a benchmark UN report for policymakers predicted, top scientists said ahead of international climate talks starting Monday in Poland.

Evidence published since the Intergovernmental Panel for Climate Change's (IPCC) February 2007 report suggests that future global warming may be driven not just by things over which humans have a degree of control, such as burning fossil fuels or destroying forest, a half-dozen climate experts told AFP.

Even without additional drivers, the IPCC has warned that current rates of greenhouse gas emissions, if unchecked, would unleash devastating droughts, floods and huge increases in human misery by century's end.

But the new studies, they say, indicate that human activity may be triggering powerful natural forces that would be nearly impossible to reverse and that could push temperatures up even further.

At the top of the list for virtually all of the scientists canvassed was the rapid melting of the Arctic ice cap.

"In the last couple of years, Arctic Sea ice is at an all-time low in summer, which has got a lot of people very, very concerned," commented Robert Watson, Chief Scientific Advisor for Britain's department for environmental affairs and chairman of the IPCC's previous assessment in 2001.

"This has implication's for Earth's climate because it can clearly lead to a positive feedback effect," he said in an interview.

When the reflective ice surface retreats, the Sun's radiation -- heat -- is absorbed by open water rather than bounced back into the atmosphere, creating a vicious circle of heating.

"We had always known that the Arctic was going to respond first," said Mark Serreze of the National Snow and Ice Data Center in Boulder, Colorado. "What has us puzzled is that the changes are even faster than we would have thought possible," he said by phone.

New data on the rate at which oceans might rise has also caused consternation.

"The most recent IPCC report was prior to ... the measurements of increasing mass loss from Greenland and Antarctica, which are disintegrating much faster than IPCC estimates," said climatologist James Hansen, head of NASA's Goddard Institute for Space Studies in New York.

Unlike the Arctic ice cap, which floats on water, the world's two major ice sheets -- up to three kilometers (two miles) thick -- sit on land.

The IPCC's most recent assessment "did not take into account the potential melting of Greenland, which I think was a mistake," said Watson, the former IPCC chairman.

Were Greenland's entire ice block to melt, it would lift the world's sea levels by almost seven meters (22.75 feet), while western Antarctica's ice sheet holds enough water to add six metres (20 feet).

Neither of these doomsday scenarios is on the foreseeable horizon.

But for coastal dwellers, even a relatively small loss of their ice could prove devastating.

IPCC estimates of an 18-to-59 centimetre (7.2-to-23.2 inches) rise by 2100 has been supplanted among specialists by an informal consensus of one metre (39 inches), said Serreze.

The accelerating concentration of greenhouse gases in the atmosphere, and signs of the planet's dwindling ability to absorb them, are also causing some scientists to lose sleep.

During the 1970s, there were on average 1.3 parts per million (ppm) of carbon dioxide -- the main greenhouse gas -- in the air. In the 1980s the figure was 1.6 ppm, and in the 1990s 1.5 ppm.

In the period 2000-2007, however, the concentration jumped to an average 2.0 ppm, with a high of 2.2 last year, according to the Global Carbon Project, based in Australia.

"The present concentration is the highest during the last 650,000 years and probably during the last 20 million years," said the Global Carbon Project's Pep Canadell, a researcher at Australia's Commonwealth Scientific and Industrial Research Organisation.

And in 2008, he said, there has been an "exponential growth" in the atmospheric concentration of methane, another greenhouse gas that is an even more potent driver of global warming than CO2.

One potential source of both gases is frozen tundra in the Arctic and sub-Arctic regions, where temperatures have risen faster than anywhere else on Earth.

"The amount of carbon that is locked up in permafrost that could be released into the atmosphere is just about on a par with the atmospheric load the world has right now," said Serreze.

These higher concentrations of greenhouse gases come at a time when Earth's two major "carbon sinks" -- forests and especially oceans -- are showing signs of saturation.

The December 1-12, 2008, forum of the 192-member UN Framework Convention on Climate Change (UNFCCC) comes midway through a two-year process launched in Bali for braking the juggernaut of global warming.

Scheduled to run until December 12, the talks are a stepping stone towards a new pact -- due to be sealed in Copenhagen in December 2009 -- for reducing emissions and boosting adaptation funds beyond 2012, when the current provisions of the UN's Kyoto Protocol expire.

New rifts have developed on the Wilkins Ice Shelf that could lead to the opening of the ice bridge that has been preventing the ice shelf from disintegrating and breaking away from the Antarctic Peninsula.

The ice bridge connects the Wilkins Ice Shelf to two islands, Charcot and Latady. As seen in the Envisat image above acquired on 26 November 2008, new rifts (denoted by colourful lines and dates of the events) have formed to the east of Latady Island and appear to be moving in a northerly direction.

Dr Angelika Humbert from the Institute of Geophysics, Münster University, and Dr Matthias Braun from the Center for Remote Sensing, University of Bonn, spotted the newly formed rifts during their daily monitoring activities of the ice sheet via Envisat Advanced Synthetic Aperture Radar (ASAR) acquisitions.

"These new rifts, which have joined previously existing rifts on the ice shelf (blue dotted line), threaten to break up the chunk of ice located beneath the 21 July date, which would cause the bridge to lose its stabilisation and collapse," Humbert explained. "These recent changes are happening slower and more continuously than the events we saw earlier this year."

Wilkins Ice Shelf: 20-26 November

In February 2008 an area of about 400 km² broke off from the ice shelf, narrowing the ice bridge down to a 6 km strip. At the end of May 2008 an area of about 160 km² broke off, reducing the ice bridge to just 2.7 km. Between 30 May and 9 July 2008, the ice shelf experienced further disintegration and lost about 1 350 km².

The Wilkins Ice Shelf, a broad plate of floating ice south of South America on the Antarctic Peninsula, had been stable for most of the last century before it began retreating in the 1990s. The peninsula has been experiencing extraordinary warming in the past 50 years of 2.5°C.

If the ice shelf breaks away from the peninsula, it will not cause a rise in sea level since it is already floating. However, ice shelves on the Antarctic Peninsula are sandwiched by extraordinarily raising surface air temperatures and a warming ocean, making them important indicators for on-going climate change.

Long-term satellite monitoring over Antarctica is important because it provides authoritative evidence of trends and allows scientists to make predictions. Over the last 17 years, ESA’s ERS and Envisat satellite missions have been the main vehicles for testing and demonstrating the use of Earth Observation data in Polar Regions.

Break-ups of Larsen-B and Wilkins ice shelves

In the past 20 years, seven ice shelves along the Antarctic Peninsula have retreated or disintegrated, including the most spectacular break-up of the Larsen B Ice Shelf in 2002, which Envisat captured within days of its launch.

Envisat’s ASAR instrument is particularly suited to acquire images over Antarctica during the local winter period because it is able to produce high-quality images through bad weather and darkness, conditions often found in the area.

Daily ASAR images of Antarctica are easily accessible to scientists. ESA will publish an update about the status of the Wilkins Ice Shelf in the event of a break-up.

Friday, November 28, 2008

Poznań, Poland, is hosting the UN Climate Change Conference this year. This venue happens to be only about a 100 miles north of where I currently reside, and some of the readers of this blog actually do live in Poznań (well at least while they’re there for medical school). In any case the reason why I’m bringing all this up is a new study by WWF. Basically it says that if the global temperature rises by 2 degrees the Arctic and Greenland ice covers will melt. That in turn will result in a sea level rise of possibly more than 10 meters. I leave the results of this scenario up to your imagination. Here is the WWF article:

“Sea levels set to rise faster than expected”

Geneva, Switzerland: Even warming of less than 2°C might be enough to trigger the loss of Arctic sea ice and the meltdown of the Greenland Ice Sheet, causing global sea levels to rise by several meters.

Ahead of next week’s meeting of governments in Poznan, Poland, for UN climate talks, WWF analysis of the latest climate science comes to the dire conclusion that humanity is approaching the last chance to keep global warming below the danger threshold of 2°C.

”The latest science confirms that we are now seeing devastating consequences of warming that were not expected to hit for decades,” said Kim Carstensen, WWF Global Climate Initiative leader.

“The early meltdown of ice in the Arctic and Greenland may soon prompt further dangerous climate feedbacks, accelerating warming faster and stronger than forecast.

“Responsible politicians cannot dare to waste another second on delaying tactics in the face of these urgent warnings from nature.

“The planet is now facing a new quality of change, increasingly difficult to adapt to and soon impossible to reverse.

“Governments in Poznan must agree to peak and decline global emissions well before 2020 to give people reasonable hope that global warming can still be kept within limits that prevent the worst.

“In addition to constructive discussions in Poznan we need to see signals for immediate action.”

The CO2 storage capacity of oceans and land surface – the Earth’s natural sinks – has been decreasing by 5 per cent over the last 50 years. At the same time, manmade CO2 emissions from fossil fuels have been increasing – four times faster in this decade than in the previous decade.

WWF is urging governments to use the Poznan talks for an immediate U-turn away from the fatal direction the world is heading in.

“We are at the point where our climate system is starting to spin out of control,” said Carstensen. “A single year is left to agree a new global treaty that can protect the climate, but the UN talks next year in Copenhagen can only deliver this treaty if the meeting in Poznan this year develops a strong negotiation text.”

Rube Goldberg is alive and well in the climate record. In an effort to explain several spikes in global carbon dioxide (CO2) levels during the last ice age, researchers have come up with the following scenario: Fresh water flooded the North Atlantic Ocean, which slowed ocean circulation, which impeded the transport of nutrients to the ocean's food web, which starved CO2-consuming organisms that form the basis of that web, which resulted in CO2 buildup in the atmosphere, which eventually heated Greenland and possibly much of the world. The biggest surprise is the probable source of the fresh water that got the whole thing started: rogue icebergs.

Setting Earth's thermostat isn't easy. It's determined by a combination of many interconnected factors, local and global, each of which can impact any or all of the others. One such factor is the huge underwater current known as the North Atlantic Deep Water--or the Conveyor Belt--which begins off Greenland and encircles the globe. The current has long been suspected of helping to drive climate change, though exactly how has remained unknown. Climate scientists have also been curious about three seemingly unrelated but simultaneous developments that occurred on and off during the last half of the most recent ice age, between 65,000 and 13,000 years ago: a drop in salinity in the North Atlantic, a distinct chemical change in ocean sedimentary layers around the world, and sudden cooling and even more sudden heating in Greenland.

Climate scientists Andreas Schmittner of Oregon State University, Corvallis, and Eric Galbraith of Princeton University developed a computer model to see if they could connect ancient climate upheavals to ocean circulation. They designed the model to mimic present-day conditions, but they dropped temperatures to ice-age levels. Then, they flooded their virtual North Atlantic with fresh water and watched what happened. "We hit our climate machine with a big hammer," Galbraith says.

As the researchers report this week in Nature, the action created the climate cascade described above, building up CO2 over a millennium and warming Greenland by as much as 16°C. Then, after a few more millennia, the process repeated, just like in the geological records.

The model shows that disruptions in the North Atlantic Deep Water are "tightly linked to CO2" in the atmosphere, Galbraith says. "This had been a vexingly difficult relationship to understand," he adds, "but these simulations show a remarkably simple way to connect them." And where did all the freshwater come from? Galbraith says the most likely source was periodic melting of migrating icebergs from northern Canada and Eurasia, which broke off from the massive polar ice sheets.

"People have been saying for years that we don't understand what makes the level of CO2 in the atmosphere go up and down during the ice ages," says oceanographer J. R. Toggweiler of the Princeton-based National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory. But the paper explains the phenomenon "very nicely by a simple change in the oceanic circulation," says Toggweiler, who was not involved in the study. In terms of applications, climate scientist Claire Parkinson of NASA's Goddard Space Flight Center in Greenbelt, Maryland, says that understanding the ocean's past cycles can help researchers predict "how much the ocean can continue to be a sink for carbon dioxide and other greenhouse gases."

GENEVA (AFP) – A less than two degree Celsius rise in global temperatures might be sufficient to spark a meltdown of the Greenland Ice Sheet and Arctic sea ice, the WWF warned in a new study released Thursday.

"Scientists now suggest that even warming of less than 2 degree Celsius might be enough to trigger the loss of Arctic sea ice and the meltdown of the Greeland Ice Sheet," the WWF said in a statement to accompany the findings.

"As a result, global sea levels would rise by several metres, threatening tens of millions of people worldwide."The melting of Arctic sea ice could affect ecosystems, while a meltdown of the Greenland Ice Sheet could lead to a sea level rise of up to seven metres, with a devastating impact for the rest of the world.

The WWF urged governments meeting for UN climate talks in Poland starting Monday to "develop a strong negotiation text for a new climate treaty" due at the end of next year.

Kim Carstensen, WWF Global Climate Initiative leader said: "The early meltdown of ice in the Arctic and Greenland may soon prompt further dangerous climate feedbacks accelerating warming faster and stronger than forecast.

"Responsible politicians cannot dare to waste another second on delaying tactics in the face of these urgent warnings from nature."

A new study predicts for the first time that a major population of Atlantic cod, near Newfoundland, Canada, will essentially go extinct within 20 years, despite best attempts to manage it. "This is the most shocking and disturbing news I've ever heard about a marine fish population," says fisheries biologist Jeffrey Hutchings of Dalhousie University in Halifax, Canada.

Atlantic cod is a symbol of boom-and-bust commercial fishing. After 50 years of heavy harvesting in the late 20th century, the Canadian cod fishery collapsed in the early 1990s. Total bans ensued, and fisheries managers expected to see a recovery. However, after 15 years of little to no fishing, local populations show no sign of rebounding. In fact, some will continue to spiral downward, according to projections reported in this month's issue of the Canadian Journal of Fisheries and Aquatic Sciences.

Biologists Douglas Swain and Ghislain Chouinard of Canada's Department of Fisheries and Oceans used well-established models of fishery stocks to predict the future of the fourth largest population of cod, in the southern Gulf of St. Lawrence, southwest of Newfoundland. The models took into account the population's productivity, based on the proportion of young fish that mature, the growth of adults, and natural mortality rates. The results were sobering: The southern Gulf cod stock will be extirpated (local extinction defined as less than 0.3% of the species' original biomass) within 20 years if limited fishing is allowed. Even if the fishery is completely closed, the stock will hit rock bottom in 38 years.

The main problem, according to Swain and Chouinard, is that adult cod have been dying at an unusually high rate in recent years. No one knows why, but Swain suspects the cause might be increased predation by seals. The problem may be more widespread: The neighboring Scotian Shelf cod population also took a nosedive in the 1990s based on data from a Canadian report published in 2003. Furthermore, while most other cod populations off Canada appear to be stable, the same could have been said about the southern Gulf population up until a few years ago, says Swain.

Although biologists have traditionally assumed that stocks will rebound if fishers simply stop fishing, Hutchings notes, the new study of cod is an "extremely compelling example of the fallacy of that assumption." As for extirpation of a cod population, Hutchings says he never considered it possible until this analysis. However, fisheries biologist Ralph Mayo of the National Oceanic and Atmospheric Administration in Woods Hole, Massachusetts, says the outlook could be better for smaller U.S. cod stocks in the Gulf of Maine and on Georges Bank. "The Gulf of Maine population has even been increasing," he says. That, of course, is small consolation for Canada.

Thursday, November 27, 2008

The Louisiana coastline could feel the impacts of hurricanes, even those that don't make landfall. (Credit: Image courtesy of Global Warming Art)

ScienceDaily (Nov. 24, 2008) — In “Dover Beach,” the 19th Century poet Matthew Arnold describes waves that “begin, and cease, and then again begin…and bring the eternal note of sadness in.”

But in the warming world of the 21st Century, waves could be riding oceans that will rise anywhere from 0.5 meters (19 inches) to 1.4 meters (55 inches), and researchers believe there’s a good chance they will stir stronger feelings than melancholia.

Several scientists from Scripps Institution of Oceanography at UC San Diego are finding that sea level rise will have different consequences in different places but that they will be profound on virtually all coastlines. Land in some areas of the Atlantic and Gulf coasts of the United States will simply be underwater.

On the West Coast, with its different topography and different climate regimes, problems will likely play out differently. The scientists’ most recent conclusions, even when conservative scenarios are involved, suggest that coastal development, popular beaches, vital estuaries, and even California’s supply of fresh water could be severely impacted by a combination of natural and human-made forces.

Scripps climate scientists often consider changes in average conditions over many years but, in this case, it’s the extremes that have them worried. A global sea level rise that makes gentle summer surf lap at a beachgoer’s knees rather than his or her ankles is one thing. But when coupled with energetic winter El Niño-fueled storms and high tides, elevated water levels would have dramatic consequences.

The result could transform the appearance of the beaches at the heart of California’s allure.

Sea level has been trending upward for millennia. For the last 6,000 years, it is estimated that global sea levels have rising an average of five centimeters (2 inches) per century. Before that, between 18,000 and 6,000 years ago, the seas rose a full 120 meters (400 feet). Step by step, they bit into rocky coastlines like California’s by smashing cliffs, creating beaches with the debris, rising a bit more, and repeating the process over and over again.

Humans are speeding up the pace of that assault. The United Nations-sponsored Intergovernmental Panel on Climate Change (IPCC) reported that sea level rose, on average, 1.7 millimeters (0.07 inches) per year over the entire 20th Century. But recent estimates from satellite observations find a marked increase, at 3.1 millimeters (0.12 inches) per year since 1993.

The oceans are rising because the warming ocean water increases in volume and because water is being added from melting glaciers and land-based ice sheets. The complex difficult-to-predict contribution of the latter is such a matter of controversy that the recent IPCC Fourth Assessment report didn’t factor glacial melt into its sea level rise estimates. Today there is quite broad-based opinion that the IPCC estimates are considerably lower than the higher range of possible sea level rise. Some individuals, pointing to the quantity of water frozen in Greenland and Antarctica and to ancient sea level evidence, have suggested that sea level rise could reach several meters by the end of the 21st Century. However, an August paper in the journal Science co-authored by former Scripps postdoctoral researcher Shad O’Neel suggests that some of the more exaggerated claims that water could rise upwards of 10 meters (33 feet) by century’s end are not in the realm of possibility. O’Neel and co-authors indicate that the realities of physics impose a cap of 2 meters (6.6 feet) for possible sea level rise by 2100.

“That’s fine,” said Scripps climate researcher Dan Cayan, who is leading an analysis of climate change scenarios for the state of California, “but two meters is still enough to do a lot of damage.”

Recent news footage of overtopped levees makes it easy to envision what two meters’ difference means to low-lying cities like New Orleans, especially when extreme events like hurricanes are factored in. Any flooding would be proportionately higher than it is now. Additionally Bromirski recently showed that sea level rise will amplify the power and frequency of hurricane-generated waves that reach shore, even if the storms themselves don’t make landfall.

In contrast to the beaches of the East Coast, many of which are covered with vast expanses of sand, California’s coastline is predominantly bedrock covered by a relatively thin veneer of sand. That sand can shift or disappear during storms. Thus, preserving the precious supply that keeps the tourists coming has for decades been a priority for state officials. Resource management, however, has required them to make trade-offs. They have constructed seawalls to protect houses built on ocean cliffs. They have dammed rivers to create supplies of water for drinking and to prevent floods and debris from damaging downstream developments.

In so doing, nature’s two primary sources of beach replenishment have been muted in a process known as passive erosion. Managers have compensated through artificial beach replenishment projects but at a costs that approach $10 per cubic yard. Since usually millions of cubic yards of sand need to be moved, there are monetary limits to what they can reasonably accomplish.

Reinhard Flick, who received his doctorate in oceanography from Scripps in 1978, needs only to look out his office window to watch the losing battle of beaches unfold. During his student days, he used to play volleyball on stretches of sand that are now underwater except during low tide. Rocks buried under several feet of sand four decades ago are now exposed for large parts of the year.

The staff oceanographer for the California Department of Boating and Waterways, Flick said that seawalls causing passive erosion will likely combine with sea level rise to doom some Southern California beaches. The change will become most apparent during El Niño events, when a pool of warm Pacific Ocean water settles off the coast for a year or two. El Niño has a dual effect on the West Coast. It not only feeds more intense storms but the warm ocean water itself causes a temporary spike in sea level that is above and beyond the rise that climate change is causing. During the 1997-98 El Niño, for instance, tide gauges off San Francisco recorded that sea level was 20 centimeters (8 inches) above normal for more than a year, including the winter storm season. That temporary rise is about equal to the rise observed for the entire 20th Century.

If sea levels rise substantially, when a large storm coincides with a high tide during an El Niño event, there could be widespread inundation along the California coast. Effects could range from a submersion of areas of San Diego’s Mission Beach to an inundation of the Sacramento-San Joaquin Delta. There, an overtopping of the delta’s levees by brackish water could paralyze the main component of the state’s water delivery system. Cayan noted that repairs to the system could take months.

The threat resonates with state officials, who have tasked Scripps and other institutions with creating and updating sea level rise scenarios.

“There’s no clear path forward with sea level rise,” said Tony Brunello, deputy secretary for climate change and energy at the California Resources Agency, a key Scripps partner in developing the state’s response to manifestations of global warming. “You typically want to work with one number (but) what we want people to do is work with the whole range of estimates.”

Cayan and other Scripps researchers who are collaborating to study sea level rise emphasize that there remains a great deal of uncertainty in the creation of estimates for the coming century. The range of rise estimated by Cayan is based on scenarios of global air temperatures over the next 100 years, which range from about 2° C (3.6° F) to about 6° C (10° F). By 2100, global sea level rise reaching a half-meter seems likely, and if the higher rates of potential warming occur it could rise by more than one meter. The potential cost of any government project or policy change puts a high premium on narrowing this range. As O’Neel and his co-authors observed in their paper, the cost of raising Central Valley levees only 15 centimeters (6 inches) to prepare for higher sea levels has been estimated at more than $1 billion.

“These are very broad-brush preliminary kinds of studies right now, but you have to start somewhere,” said Scripps coastal oceanographer Bob Guza.

Flick said it will be essential for scientists to be able to study the effects of the next El Niño so they can begin to understand not just where damage will happen on the California coast but to what extent. He only had surveyor’s equipment and aerial photos available to him to measure beach changes after the 1982-83 El Niño, but Guza and his collaborators now have light detection and ranging (LIDAR) and GPS technologies to make precise surveys of beach and cliff damage. Guza and Flick hope that Scripps can not only enhance its use of such technology but to deploy it within hours of a major storm event.

“We need to be geared up to quantify what beach changes are,” said Flick. “We have to do an even better job of studying wave forces and wave climate.”

If there’s any good news for Southern California, Scripps climate scientist Nick Graham has estimated that ocean warming trends will drive storm tracks farther north, perhaps sparing the state’s lower half from the full brunt of buffeting El Niño waves the 21st Century will generate. Graham compared winds produced in three different simulations of climate change with those generated in the late 20th Century. The models showed that Southern California can expect a moderate decrease in wave size of about 0.25 meters (10 inches). But even there, Graham sees a problem.

Wednesday, November 26, 2008

Changing winds affect ice shelves

Regional wind-forcing plays a critical role in controlling delivery of ocean heat to Amundsen Sea ice shelves. So say researchers at the British Antarctic Survey (BAS) who have modelled the circulation in this region to understand what drives relatively warm Circumpolar Deep Water (CDW) onto the continental shelf. The CDW is responsible for the high melt rates observed underneath the floating tongues of the major outlet glaciers that drain into Pine Island Bay.

The new result is significant, explains team member Adrian Jenkins, because the timescales needed for atmospheric warming to penetrate key water masses are centuries or longer, which would suggest that ice sheets are relatively immune to recent climate change. However, if temperature increases in the atmosphere are accompanied by changes in atmospheric circulation, then ice sheets could be affected much sooner. The model suggests that any changes in the atmosphere, be they natural or anthropogenic, that alter regional winds will affect how ocean heat is delivered to Antarctica's ice shelves and floating glacier tongues.

The researchers used a version of the Miami Isopycnic Coordinate Ocean Model adapted for domains that include ice shelves. This was coupled to a dynamic/thermodynamic sea ice model, forced with surface pressure – from which surface winds were derived – and temperature from NCEP/NCAR reanalyses.

Jenkins stresses, however, that the results are a simply a "hindcast". "We have offered one possible explanation for glaciological changes that have been observed," he told environmentalresearchweb. "However, we have not made any predictions about the future and have stressed that our results show decadal variability rather than a long-term trend."

He adds that it is not yet possible to extract a trend from the variability or say whether it is entirely natural or related to anthropogenic forcing.

The Amundsen Sea is an important location for climate change studies. Indeed, a US cruise early next year, led by team member Stan Jacobs of the Lamont-Doherty Earth Observatory, will continue oceanographic observations on the Amundsen Sea Continental Shelf and deploy instruments for year-round ocean monitoring. The UK will participate thanks to the Natural Environment Research Council's autonomous underwater vehicle, Autosub-III, which will be used to directly measure ocean properties beneath the floating ice shelves, explains Jenkins.

"These results should provide us with more information on the variability of ocean forcing on ice shelves and the processes by which the warmth of ocean waters leads to melting," he adds. "This will help improve our model representation of the continental shelf and floating ice shelves."

The team is now running its model at higher resolution and extending the length of the integrations.

Ice in the environment, whether in the form of ice particles in clouds or sea ice and snow at the Earth's surface, has a profound influence on atmospheric composition and climate. The interaction of trace atmospheric gases with snow and sea ice surfaces largely controls atmospheric composition in polar regions. The heterogeneous chemistry of ice particles in clouds also plays critical roles in polar stratospheric ozone depletion and in tropospheric chemistry. A quantitative physical understanding of the interactions of snow and ice with trace gases is critical for predicting the effects of climate change on atmospheric composition, for the interpretation of ice core chemical records, and for modeling atmospheric chemistry.

The motivation behind this focus issue of Environmental Research Letters (ERL), and the special session at the Fall 2007 meeting of the American Geophysical Union that generated it, was to enhance communication and interactions among field and laboratory scientists and modelers working in this area. Members of these three groups are each working toward a mutual goal of understanding and quantifying the connections between the chemistry of snow and ice in the environment and atmospheric composition, and communication and collaboration across these traditional disciplinary boundaries pose a challenge for the community.

We are pleased to present new work from several current leaders in the field and laboratory communities in this focus issue. Topics include the interaction of organics and mercury with snow and ice surfaces, halogen activation from halide ice, and the emissions of reactive nitrogen oxides from snow. Novel experimental techniques are presented that make progress towards overcoming the experimental challenges of quantifying the chemistry of realistic snow samples and ice chemistry at temperatures relevant to the polar boundary layer. Several of the papers in this issue also touch on one of the significant gaps in our current understanding of the atmospheric chemistry of ice: the role of a quasi-liquid layer (QLL) or quasi-brine layer (QBL) at the ice surface.

The studies presented here advance our understanding of the complex interactions of snow and ice with important reactive components in our atmosphere. It has become clear in recent years that the polar regions do not act as an ultimate sink for many compounds—the release of halogens and reactive nitrogen oxides from ice and snow are examples of this. Two notable implications arise from these findings (i) the impact of anthropogenic pollutants in our environment may extend further than we fully appreciate with current global atmospheric chemistry models and (ii) our interpretation of chemical records in ice cores requires that we fundamentally understand and quantify air–snow and air–ice interactions. Additionally, laboratory studies are elucidating the details of heterogeneous reactions that are prevalent on ice and snow surfaces throughout the troposphere, and we are poised to make significant strides in the near future quantifying these effects on regional and global scales. We look forward to continued progress in this field in the coming years, and we will continue to work to connect those conducting modeling, field and laboratory studies.

Focus on Connections between Atmospheric Chemistry and Snow and Ice Contents

Increasingly an analogy emerges between these developments and aspects of abrupt climate changes associated with the last glacial termination.As stated by Alley et al. (2003) “Large, abrupt, and widespread climate changes with major impacts have occurred repeatedly in the past, when the Earth system was forced across thresholds.” Ice core and sedimentary evidence for the Pleistocene (1.8 Ma – 10,000 years ago) demonstrate abrupt glacial terminations, intra-glacial global warming events (Dansgaard–Oeschger cycles; Broecker, 2000; Ganopolski & Rahmstorf, 2002; Braun et al., 2005) as well as sharp to protracted cooling periods.The latest glacial termination includes a number of tipping points which involve sharp rise and fall of temperatures by several degrees C over time scales of centuries, decades, or even a few years (Clark et al., 2003; Kobashi et al., 2008; Steffensen et al., 2008), affecting both high latitudes and tropical zones (Hughen et al., 1996).

1.CO2 rise rates: Late 20th century and early 21st century rates averaging 1.45 ppm/yr and rising to 1.8 ppm/yr in 2006 and 2.2 ppm/yr in 2007, exceed 1850–1970 rates by factors of ~4–5 and are two orders of magnitude higher than mean CO2 rise rates of the last glacial termination (~0.014 ppm/yr) (Rahmstorf et al., 2006; Global Carbon Project, 2008).

3.Temperature rise rates:Mean temperature rise rates of 0.016 oC during 1970–2007 were about an order of magnitude higher than during 1850–1970 (0.0017 oC) and the last glacial termination. As indicated by deuterium studies of Greenland ice cores, abrupt tipping points during the last termination (14.7–11.7 kyr) resulted in extreme temperature changes on the scale of several degrees C in a few years (Steffensen et al., 2008).

4.Sea level rise rates:Mean sea level rise rate of ~0.32 cm/yr during 1988–2007 more than doubled relative to the mean ~0.14 cm/yr rate of 1973–1988 and three times those of 1850–1970. In so far as doubling of sea level rise rates continues at this rate through the 21st century, they may approach rates similar to those of the last glacial termination (1.3–1.6 cm/yr) before mid-century, with sea level rise by several metres toward the end of the century as estimated by Hansen et al. (2007).

Whereas larger ice sheets existed on Earth at the outset of the last glacial termination, when the large Laurentian and Fennoscandian ice sheets began to melt, than during the Holocene, comparisons between climate forcings during the glacial termination and those operating since about 1750 may be instructive:

1. The last glacial termination, triggered by insolation peaks, involved total radiative forcing rise of about 6.5 Watt/m2, including ~3.0±0.5 Watt/m2 induced by rising greenhouse gases (GHG: CO2, CH4, NxO) and 3.5±1.0 Watt/m2 induced by lowered albedo associated with melting of ice sheets and spread of vegetation. Both factors, including their feedback effects, result in mean global temperature rise of ~5.0±1.0 oC (Hansen et al., 2008).

2. Since about 1750 global warming is driven by radiative GHG forcing of near + 3.0 Watt/m2 consequent on rise of GHG (CO2, CH4, NxO, ozone, halocarbons), compensated in part by albedo increase due to land clearing (–0.2 Watt/m2), aerosols (–0.5 Watt/m2) and clouds (–0.7 Watt/m2).When the albedo loss due to melting of the Arctic and Antarctic sea ice, the margins of Greenland and Antarctic ice sheets and mountain glaciers, is accounted for, the total forcing would be tracking toward values about half those of the last glacial termination of 6.5±1.5 Watt/m2.

Detailed deuterium proxy-based paleo-temperature studies of Greenland ice cores GISP-2 indicate that, far from smooth, the transitions associated with the glacial terminations involved abrupt tipping points where temperatures rose or fell sharply by several degrees C over time scales as short as a few decades or even a few years (Kobashi et al., 2008; Steffensen et al., 2008).A potential onset of such tipping points in the context of 21st century climate change is consistent with observations pertaining to the last glacial termination, currentmethane release from sediments off-shore Siberia and from permafrost, Arctic Sea ice melt, Antarctic sea ice and ice shelf melt and intensifying Atlantic hurricanes.

A marked climate tipping point is defined about 1975–76, with abrupt rise of temperature and temperature rise rates. 1975–2008 climate change developments incurred CO2 rise by 55 ppm (332–387 ppm) and mean temperature rise of ~0.9 oC for the Northern Hemisphere (mean CO2 rise ~1.7 ppm/yr; temperature rise 0.027 oC/yr; 0.016 oC per 1 ppm CO2). In so far as the relations between CO2 and temperature during 1975–2008 can be used as a baseline, a rise of CO2 levels to 450 ppm by 2050 would result in minimum additional temperature rise by approximately 1.0 oC relative to 2008.

Wigley (1993, 2006) and Wigley et al. (2007) modeled CO2 trajectories, accounting for carbon feedbacks, reversal of atmospheric CO2 overshoots and stabilization, stating: “Stabilization of the climate system requires stabilization of greenhouse-gas concentrations. Most work to date has considered only stabilization of CO2, where there are choices regarding both the concentration stabilization target and the pathway towards that target. Here we consider the effects of accounting for non-CO2 gases (CH4 and N2O), for different CO2 targets and different pathways. As primary cases for CO2 we use the standard “WRE” pathways to stabilization at 450 ppm or 550 ppm. We also consider a new “overshoot” concentration profile for CO2 in which concentrations initially exceed and then decline towards a final stabilization level of 450 ppm, as might occur if an initial target choice were later found to be too high.”

Principal alternatives considered in the Garnaut (2008) Climate Change Review include (p. 277):

1. “Australia’s full part for 2020 in a 450 scenario would be a reduction of 25% in emissions entitlements from 2000 levels, or one-third from Kyoto compliance levels over 2008–2012, or 40% per capita from 2000 levels. For 2050, reductions would be 90% from 2000 levels (95% per capita)”.

2. “Australia’s full part for 2020 in a 550 scenario would be a reduction in entitlements of 10% from 2000 levels, or 17% from Kyoto compliance levels over 2008–2012, or 30% per capita from 2000. For 2050, reductions would be 80% from 2000 levels or 90% per capita.”

3. “If there is no comprehensive global agreement at Copenhagen in 2009, Australia, in the context of an agreement among developed countries only, should commit to reduce its emissions by 5% (25% per capita) from 2000 levels by 2020, or 13% from the Kyoto compliance 2008–2012 period.”

The differences between the 550 ppm and 450 ppm scenarios are as follows (p.86):

No-mitigation case.A global emissions case in which there is no action to mitigate climate change—the Garnaut–Treasury reference case—was developed as part of the Review. This emissions case recognizes recent high trends in the emissions of carbon dioxide and other greenhouse gases. Emissions continue to increase throughout the 21st century, leading to an accelerating rate of increase in atmospheric concentrations. By the end of the century, the concentration of long-lived greenhouse gases is 1565 ppm CO2-e, and carbon dioxide concentrations are over 1000 ppm—more than 3.5 times higher than pre-industrial concentrations.

550 mitigation case.Emissions peak and decline steadily, so that atmospheric concentrations stop rising in 2060 and stabilize at around 550 ppm CO2-e—one-third of the level reached under the no-mitigation case.450 mitigation case.Emissions are reduced immediately and decline more sharply than in the 550 case. Atmospheric concentrations overshoot to 530 ppm CO2-e in mid-century and decline towards stabilization at 450 ppm CO2-e early in the 22nd century.”

The Review (p. 95) acknowledges: “The small change in global average temperature between the 550 and 450 mitigation pathways could have a relatively large impact on sea-ice extent.”Yet Table 11.1 and Figure 44 of Garnaut-2008 Review, suggest the difference by 2050 is no more than 0.1 oC:

1. A CO2-e rise of 450 ppm by 2050 would raise temperature by +1.6 oC relative to 1990

2. A CO2-e rise of 550 ppm by 2050 would raise temperature by +1.7 oC relative to 1990.

As indicated above, these estimates are near one order of magnitude low as compared to projections based on climate sensitivity, estimated at 3±1.5 oC per doubling of CO2 concentration (Charney, 1979).

2.Garnaut-2008’s choice between a 450 ppm and 550 ppm trajectory for 2050, projected difference of 0.1 oC per 100 ppm CO2-e rise for these trajectories, and the assumption of CO2 ‘stabilization,’ are difficult to reconcile with extensions of the 1975–2008 CO2 trend. These projections take little account of the consequences of non-linear climate feedback processes due to methane release from sediments and permafrost, ice sheet breakup, infrared absorption by exposed sea water, and consequent climate tipping points.

3.The assumption that CO2 levels can be reversed from 550 ppm, once reached, to 450 ppm over acceptable time scales, finds little support in the centuries-scale atmospheric residence time of CO2 and in past atmospheric records.

Climate models, effective in modeling 20th and early 21st century climate change, tend to underestimate the magnitude and pace of global warming (Rahmstorf et al., 2007). According to Hansen et al. (2008) “Climate models alone may be unable to define climate sensitivity more precisely, because it is difficult to prove that models realistically incorporate all feedback processes. The Earth’s history, however, allows empirical inferences of both fast feedback climate sensitivity and long term sensitivity to specified greenhouse gas change including the slow ice sheet feedback.”